Synthesis And Properties Of Heterocyclic Fluorescent Compounds

Organic small molecule fluorescent probes are an important way to detect heavy metal ions,which has advantages such as simple synthesis,easy modification,high sensitivity,no pretreatment and convenience.In this paper,a series of new heterocyclic compounds with a furan ring interlinked with thiadiazole or oxadiazole were synthesized,of which the fluorescence and Pd²⁺detection properties were studied.Starting from furoic acid and furan dicarboxylic acid,bicyclic and tricyclic compounds with a furan ring interlinked with thiadiazole or oxadiazole,such as2-amino-5-?2-furanyl?-1,3,4-thiadiazole?2?,2-amino-5-?2-furyl?-1,3,4-oxadiazole?5?and 2-?2-furyl?-1,3,4-oxadiazole?6?,2,5-furanbis?1,3,4-thiadiazole-2-amine??10?and2,5-furanbis?1,3,4-oxadiazole-2-amine??13?,were synthesized via direct condensation or indirect condensation of acylhydrazine.Besides,Schiff base-type fluorescent compounds 7,8,14 and 15 were synthesized by the condensation of salicylaldehyde with the amino groups of compounds 2,5,10 and 13.All the synthesized compounds were characterized by UV-Vis,FT-IR,ESI-MS or NMR etc.Fluorescence performance studies show that Schiff bases 7,8,14 and 15 have aggregated fluorescence characteristics,with a strong solid fluorescence but without fluorescence in solution.Compounds 2,5,6,10 and 13 with a furan ring interconnected with thiadiazole or oxadiazole have a strong fluorescence in solution but no fluorescence in solid state.UV-Vis research shows that the UV-visible absorptions of compounds 2,5,6,10and 13 were negligibly affected by Zn²⁺,Mn²⁺,Mg²⁺,Ca²⁺,Ni²⁺,Ce³⁺,Rb⁺,Y³⁺,La³⁺,Cu²⁺,Pb²⁺,La³⁺and Cu²⁺,except for Pd²⁺,indicating a good Pd²⁺selectivity.Similarly,metal ions such as Zn²⁺,Mn²⁺,Mg²⁺,Ca²⁺,Ni²⁺,Ce³⁺,Rb⁺,Y³⁺,La³⁺,Cu²⁺,Pb²⁺,La³⁺and Cu²⁺have almost no effects on the fluorescence emission spectra of compounds 2,5,6,10 and 13.However,Pd²⁺can significantly reduce or completely quench their fluorescence,which could be used for the selective detection of Pd²⁺.Compounds 2,5,6,10 and 13 have a linear Pd²⁺detection range of 0.01-0.1 uM and detection limits of 4.35×10^-7 mol/L,8.97×10^-7 mol/L,2.5×10^-66 mol/L,1.07×10^-9 mol/L and 3.97×10^-9 mol/L,respectively.Interference experiments show that anions have no effect on the detection of Pd²⁺,the suitable pH range of compounds 2,5 and 6 is 6.0-9.0,and the suitable pH range of compounds 10 and 13 is 11.0-5.0 and10.0-5.0,respectively.Generally,compounds 10 and 13 have a lower detection limit and a stronger anti-interference ability.The standard Pd²⁺detection curve of compound 10 was drawn,which showed a good accuracy with an error of about 4.97%when it was used to determine the known concentration of Pd²⁺solution.The standard Pd²⁺detection curve of compound 13 was drawn,which showed a good accuracy with an error of about 6.67%when it was used to determine the known concentration of Pd²⁺in the water of Fujiang River.The 10 and 13 were prepared.It was found that the fluorescence of the fluorescent papers of 2,5 and 6 was weak,difficult to be observed with naked eyes,and difficult to be applied practically.Interestingly,the fluorescent papers of compounds 10 and 13 have an obvious fluorescence,which could be used for the detection of Pd²⁺in environment.Compared with bicyclic compounds 2,5 and6,tricyclic compounds 10 and 13 have a stronger fluorescence,a lower detection limit and a higher sensitivity for detecting Pd²⁺,which have the advantages of high sensitivity,good selectivity,good accuracy,short response time and simplicity and could be used for the detection of Pd²⁺in environment.Fluorescence titration,IR,and UV jobs curves show that the action of compounds 2,5,6,10 and 13 between Pd²⁺is a coordination process.The coordination ratios of compounds 2,5,6 and 13 to Pd²⁺are 1:1,and the coordination ratio of compound 10 to Pd²⁺is 2:1.Density functional theory shows that the coordination of Pd²⁺reduces the coplanarity and conjugation of compounds 2,5,6,10and 13,and hinders the intramolecular charge transfer of compounds 2,5,6,10 and13,leading to the fluorescence quench of compounds 2,5,6,10 and 13.In addition,the energy difference between HOMO and LUMO decreases after complexation,which is consistent with the red shift phenomena of UV-Vis and fluorescence.